Fiberoptic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through dense wavelength division multiplexing (DWDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
Transmitter lasers used in DWDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels. Continuously tunable external cavity lasers have been developed to overcome this problem.
The advent of continuously tunable telecommunication lasers has introduced additional complexity to telecommunication transmission systems. Particularly, the tuning aspects of such lasers involve multiple optical surfaces that are sensitive to contamination and degradation during use. Heretofore, no systems have been available which provide adequate protection for continuously tunable telecommunication lasers. This lack has resulted in increased costs and decreased performance lifetimes for such lasers.
The invention provides a telecommunication laser apparatus in hermetically sealed containers and laser methods using hermetically sealed laser systems. In its most general terms, the apparatus of the invention comprises an external cavity laser, and a hermetically sealable container configured to enclose the external cavity laser in an inert atmosphere. The external cavity laser may be tunable by various mechanisms to allow transmission at multiple selectable wavelength channels.
The external cavity laser may comprise a gain medium and an end mirror. The gain medium may comprise a diode emitter chip including first and second output facets, with an anti-reflective coating on the second output facet. The first output facet and the end mirror define an external cavity, with the gain medium emitting a coherent beam from the second output facet along an optical path in the external cavity to the end mirror. A channel selector or like tunable element may be positioned within the external cavity in the optical path between the end mirror and the emitter chip.
The hermetically sealable container encloses the external cavity laser, including the end mirror, gain medium, and tuning element. Hermetically sealing the external cavity laser under an inert atmosphere protects the anti-reflective (AR) coating on the gain medium, as well as surfaces on the channel selector and other optical components. The deposition of contaminants onto the optical surfaces of components associated with an external cavity laser, which may occur in the absence of hermetic sealing, causes aberrations which hinder the performance of the external laser cavity and promote degradation of critical optical surfaces. Hermetically sealing the external cavity laser as provided by the invention allows for a controlled environment that prevents corrosion and contamination of the optical surfaces of the laser.
Tunable external cavity lasers usable with the invention may also comprise a grid generator, a tuning assembly configured to adjust the end mirror and the channel selector, and various other components involved in tuning and laser operation. Many of these components can have high outgassing characteristics during laser operation such that volatile hydrocarbons can contaminate and/or cause degradation of various optical surfaces of the external cavity laser. In this regard, the external cavity laser is configured to minimize or eliminate problems associated with outgassing by lubricants, adhesives, cable insulators and other components which contain volatile compounds and residual moisture by careful material selection and minimizing the use of potentially outgassing materials.
In certain embodiments, one or more activated carbon drains are sealed within the hermetically sealed enclosure and positioned to collect volatile hydrocarbons produced by outgassing from components of the external cavity laser. The activated carbon drain has a large surface area of activated carbon that allows for adsorbing or trapping the outgassing volatile organic compounds that occur during the operation of the laser. Organic hydrocarbon materials released from epoxies and lubricants used during the assembly of the external cavity laser or utilized in sealing the hermetically sealable enclosure are also trapped by the activated carbon drain. The activated carbon drain allows the optical surfaces of the tunable external cavity laser to remain free of organic contaminants in the hermetically sealed enclosure that would otherwise hinder performance.
In other embodiments, one or more moisture traps may be included within the hermetically sealable container and positioned to collect water vapor that may outgas from polyimide or other moisture holding insulator or material present in the external cavity laser. Such outgassed water vapor, if not trapped, may condense on critical optical surfaces and reduce performance of the external cavity laser, and may promote corrosion of components. Moisture condensation is particularly a concern after xe2x80x9ccool-downxe2x80x9d periods when the laser has not been in use. The material of the moisture trap may comprise a variety of desiccants. The moisture trap prevents condensation of water on optical surfaces and elsewhere that would otherwise reduce performance in the operation of the external cavity laser and promote corrosion of laser components within the hermetically sealed enclosure.
In one embodiment, the inert atmosphere sealed within the hermetically sealed container comprises nitrogen. Other inert gases may also be enclosed in the hermetically sealed enclosure such as helium, argon, krypton, xenon, or various mixtures thereof, including a nitrogen-helium mix, a neon-helium mix, a krypton-helium mix, or a xenon-helium mix. Helium may be added to the inert atmosphere to allow for testing and monitoring the level of hermeticity of the sealed container. The inert gas or gas mix included within the hermetically sealed container may be selected for a particular refractive index or other optical property.
The apparatus of the invention may also comprise a sacrificial surface located within the hermetically sealed enclosure in which both condensation and volatile hydrocarbons from outgassing are trapped upon to avoid contamination of the optical services of the tunable external cavity laser. The sacrificial surface is configured to remain cooler than surrounding surfaces during laser operation, and may be actively cooled by a cooling source and/or be made of material which provides passive cooling by acting as a heat sink which will attract volatile hydrocarbons and water vapor.
In certain embodiments, selective heating of important optical surfaces may be employed to prevent condensation of contaminants thereon. Such heating may be employed during cool-down periods when the gain medium is not powered, to prevent condensation when the external cavity laser is not in use. One or more heat sources, either positioned internally or externally to the hermetically sealed enclosure, may be used to heat the gain medium of the external cavity laser when the gain medium is not powered, in order to maintain a relatively high temperature for the anti-reflective coating on the output facet of the gain medium to prevent condensation thereon when the laser is not in use. Heating in this manner may also be used in connection with the end mirror, tunable elements or other components with sensitive optical surfaces to maintain a temperature higher than the activated carbon drain, moisture trap and/or the sacrificial surface present in the hermetically sealed container, to further prevent the contamination of these optical surfaces.
The invention may be embodied in a telecommunication laser system which comprises an optical fiber extending into the hermetic container through a hermetic fiberoptic feedthrough and optically coupled to an output facet of the external cavity laser to receive optical output therefrom. The hermetically sealable container may vary in configuration, but will generally be configured such that the optical fiber can be feed through a side of the hermetically sealable container. Various electrical leads necessary for operation of the external cavity laser, may extend into the hermetic container through hermetic feedthroughs in the sides of the hermetic enclosure.